In gyro-optical objective systems and other optical telescope applications, it is frequently desirable to scan the field of the optical objective system with a radiation detector, to determine the orientation of a radiation source relative to the optical axis of the objective system. Of the various scanning patterns available, the rosette scan is particularly advantageous in that the relatively small field of view of a radiation detector can be scanned across the entire field of the optical objective to produce a circular search pattern with a relatively large field of view.
There are a number of principal disadvantages to prior art methods of rosette scan generation. Because the pattern is the result of the addition of two concentric co-rotating or counter-rotating vectors, the magnitude of each must be established accurately by fixed mechanical parameters of the separate optical components responsible. For example, a greater or smaller deflection than desired will produce an overlapped or incomplete closure of the center of the scan pattern, effects which are generally unacceptable for optimum operation of the system.
Although minor changes can be made to the cant angle of a mirror, such changes are limited to execution during the assembly process of the optical system and, in the case of a prismatic component, the deviation is a completely fixed function which permits virtually no freedom for such adjustments. Again, the use of a refractive prismatic element for even just one of the deflection components can introduce chromatic and other aberrations which are largely uncorrectable due to the rotational nature of the principal axes of these aberrations.
For similar reasons, the use of refractive elements tends to considerably restrict the wavelength regions over which the objective system may be required to operate, and introduces excessive expense when esoteric materials are used to overcome such restrictions. Additionally, the optical design becomes unduly complex when such systems embody, for mechanical or optical convenience or performance, a combination of both refractive and reflective elements of which two are involved in the process of rosette vector generation. Furthermore, if such a system is required to be gimballed, as in a missile seeker, the use of a non-gimballed prismatic element can additionally degrade the optical resolution of the total system by the introduction of largely uncorrectable comatic effects at substantial look angles.
Yet a further difficulty lies in the necessity to provide mechanical counter-rotating drive systems for the elements concerned, and in the packaging of such systems within practical dimensions. Not only is the power requirement a significant factor, but the complex electromagnetic field structure surrounding, for example, a prism drive motor, can introduce significant noise voltages into the radiation detector and its amplifying circuits which could considerably degrade the total performance of the seeker system. Electrostatic charges, developed by the high rotational speeds of the elements concerned, can also produce similar effects.
A further requirement for correct target signal processing is a reference system for deriving continuous and precise angular information concerning the positions of the vector-producing components. This may involve the use of optical or electromagnetic transducers as pick-off elements to determine the precise position and rotational velocity of an optical element drive shaft, thereby adding to the weight and complexity of the system.
It is therefore desirable to have a scanning optical system that is relatively simple in construction, light in weight, and which produces an inherently accurate scan without inducing undesirable optical aberrations, and which facilitates the reduction of data relative to the orientation of a detected target with respect to the optical axis of the system.